The look won't be a change for some users. Twitter has been testing out the new design on about 1% of people who use the site, studying their habits and making tweaks before rolling it out to everyone.

The main change, other than aesthetics, is a built-in "compose" box in the left rail of the Web page. The new location eliminates the old pop-up style compose box and appears to be a way for Twitter to encourage more users to actually tweet instead of just browse.

Twitter has said in the past that as many as 40% of its users are "lurkers," folks who read what others post but don't share anything themselves.

Users also will see their profile and header photos at the top of a rail on the left of the page. The main Twitter feed appears to be largely unchanged.

The new design had not appeared for all users as of Tuesday. Twitter didn't offer a specific timetable,